1. Field of the Invention
The present invention relates generally to a photosensor circuit for detecting a sensor output corresponding to the intensity of light (illumination), and more particularly to a photosensor circuit having a wide dynamic range and high sensitivity in detection of the intensity of light.
2. Description of the Related Art
A photosensor circuit is known which comprises a photodiode (PD) for detecting a sensor current corresponding to the intensity of light (optical signal), and a resistance load R for converting the sensor current detected by the PD into a detected voltage varying linearly with the sensor current. Thus, the known photosensor circuit detects light intensity (optical signal) as a sensor output in the form of voltage.
One example of such known photosensor circuit is shown here in FIG. 8. As shown in this figure, the known photosensor circuit is comprised of a photodiode PD, an operational amplifier OP, and a resistor R. The photodiode PD converts an optical signal L.sub.S into a sensor current I.sub.D whose intensity is proportional to the intensity of the optical signal L.sub.S. The operational amplifier OP amplifies the sensor current I.sub.D to a predetermined gain under load of the resistor R and outputs a sensor output (detected voltage V.sub.D) proportional to the sensor current I.sub.D. Thus, the optical signal L.sub.S first detected by the photodiode PD is eventually detected in the form of a detected voltage V.sub.D varying linearly with the optical signal L.sub.S.
Another conventionally known photosensor circuit includes a photodiode (PD) for generating a sensor current corresponding to the intensity of light (optical signal), and a metal-oxide-semiconductor (MOS) transistor for converting the sensor current detected by the PD into a detected voltage varying logarithmically with the sensor current. Thus, the conventional photosensor circuit detects light intensity (optical signal) as a sensor output in the form of voltage.
One example of such conventional photosensor circuit is shown in FIG. 9. As shown in this figure, the photosensor circuit 10 is comprised of a photodiode PD, an n-channel MOS transistor Q1 connected in series with the photodiode PD, an n-channel MOS transistor Q2 having a gate connected to a junction point P (sensor detection terminal) between the photodiode PD and the n-channel MOS transistor Q1, and an n-channel MOS transistor Q3 connected in series with the n-channel MOS transistor Q2.
Connected to the junction point P is an equivalent capacitor C comprising a synthesized stray capacitance caused by the relative proximity of the photodiode PD, n-channel MOS transistor Q1, n-channel MOS transistor Q2 and wires interconnecting these parts, or a capacitor formed during the semiconductor fabrication process.
The photodiode PD detects an optical signal L.sub.S and converts it into a sensor current I.sub.D whose intensity is proportional to the intensity of the optical signal L.sub.S.
The n-channel MOS transistor Q1 forms a load of the photodiode PD and converts the sensor current I.sub.D detected by the photodiode PD into a voltage so that a detected voltage V.sub.D is developed at the sensor detection terminal P.
The n-channel MOS transistor Q1 forms a MOS transistor load having a logarithmic property in a weakly inverted condition or state for a range in which the sensor current I.sub.D is small. Thus, by converting the sensor current I.sub.D detected by the photodiode PD into a detected voltage V.sub.D having a logarithmic characteristic (i.e., varying logarithmically with the sensor current I.sub.D), the n-channel MOS transistor Q1 can logarithmically deal with variations of the sensor current I.sub.D over several figures or units and thus enlarges the dynamic range of a sensor output (detected voltage V.sub.D) relative to an input (sensor current I.sub.D).
The n-channel MOS transistor Q2 forms an output transistor and performs voltage-to-current conversion so that the detected voltage V.sub.D can be taken out from the photosensor circuit 10 in the form of a sensor current signal.
The n-channel MOS transistor Q3 forms a switch for selectively connecting and disconnecting the sensor current signal converted by the n-channel MOS transistor Q2, to and from an external circuit (not shown).
The photosensor circuit 10 of the foregoing construction operates as follows:
The n-channel MOS transistor Q1 has a drain D and a gate G both connected to a common power supply VD (5-volt, for example). When no optical signal L.sub.S is detected (i.e., when the photodiode PD is not operated), a charge current I.sub.J flows from the power supply VD through the n-channel MOS transistor Q1 to the capacitor C and thus charges the capacitor C. Accordingly, the detected voltage V.sub.D appearing at the sensor detection terminal P can rise only to a predetermined value near the power supply voltage VD. The predetermined voltage value represents the initial condition in which the photodiode PD detects no optical signal L.sub.S.
The predetermined value of the detected voltage V.sub.D in the initial condition is set to a smaller value than the power supply voltage VD because as the detected voltage V.sub.D at the sensor detection terminal P increasing with the charge of the capacitor C approaches the power supply VD, a gate-source voltage V.sub.GS (equal to the drain-source voltage V.sub.DS) of the n-channel MOS transistor Q1 is cut down to cause the drain-source impedance to increase rapidly to thereby reduce the charge current I.sub.J.
While the photosensor circuit 10 is in the initial condition, the photodiode PD detects an optical signal L.sub.S whereupon a sensor current I.sub.D flows through the photodiode PD, and so the detected voltage V.sub.D at the sensor detection terminal P decreases from the predetermined value logarithmically as a function of the drain-source impedance of the n-channel MOS transistor Q1 as the optical signal L.sub.L increases.
Since the sensor current I.sub.D of the photodiode PD is proportional to the optical signal L.sub.D, and since the detected voltage V.sub.D at the sensor detection terminal P is the product of the sensor current I.sub.D and drain-source impedance having a logarithmic characteristic, the optical signal L.sub.S can be detected by detecting an absolute value of the detected voltage V.sub.D.
FIG. 10 is a graph showing the detected voltage (V.sub.D) versus sensor current (I.sub.D) characteristic curve of the above-mentioned conventional photosensor circuit 10.
As shown in FIG. 10, while the photosensor circuit 10 is in the initial condition (sensor current I.sub.D =10.sup.-12 A), the detected voltage V.sub.D has a predetermined value of 4.5V. When the sensor current I.sub.D increases over five figures up to 10.sup.-7, the detected voltage V.sub.D becomes 4.2V.
Thus, the photosensor circuit 10 is able to detect a five-figures (one hundred thousand-fold) change in the optical signal L.sub.S by only a 0.3V change in the detected voltage V.sub.D and, hence, provides a wide dynamic range relative to the input of the optical signal L.sub.S.
However, in the conventional photosensor circuit shown in FIG. 8, detected voltages V.sub.D corresponding to optical signals L.sub.S are detected in linear characteristic. Accordingly, when the range of the optical signals L.sub.S to be detected is wide (such as five figures), the detected voltage V.sub.D becomes saturated due to restriction by the source voltage, making it difficult to increase or widen the dynamic range of the photosensor circuit.
In the case of the photosensor circuit 10 shown in FIG. 9, when the photodiode PD fails to detect the optical signal L.sub.S it is cut off to cause the charge current I.sub.J to flow into the capacitor C, thereby increasing the detected voltage V.sub.D appearing at the sensor detection terminal P. In this instance, however, due to a sudden increase in the drain-source impedance of the n-channel MOS transistor Q1 previously described, the detected voltage V.sub.D cannot exceed the predetermined value (see FIG. 10).
FIG. 11 is a graph showing the detected voltage (V.sub.D) versus time (t) characteristic curve of the conventional photosensor circuit 10 shown in FIG. 9.
As evidenced from FIG. 11, the detected voltages V.sub.D shows a sudden rise relative to the time t lapsed after the cutoff of the photodiode PD until it approaches a predetermined value (detected voltage V.sub.D =4.5). Thereafter, the detected voltage V.sub.D does not show a further increase from the predetermined value of 4.5V even when the time goes by.
Accordingly, when the photosensor circuit is employed in an indicator in which a plurality of such photosensor circuit are arranged in a matrix pattern to form a photosensor array, a difficulty arises in that due to a relatively long response time is required for the detected voltage V.sub.D to reach the predetermined value (4.5V), the indicator may hold a residual image over a relatively long period of time.
In the case of the conventional photosensor circuit 10, even in a range of the optical signal L.sub.S in which the optical signal L.sub.S has very small values (sensor current I.sub.S =10.sup.-12 -10.sup.-11 A), the detected voltage V.sub.D shows a logarithmic characteristic, as shown in FIG. 10. It is, therefore, difficult to lower a value of the minimum detectable level of the very small optical signal L.sub.S, resulting in a reduced sensor sensitivity.
Furthermore, since the n-channel MOS transistor Q1 and the capacitor C of the conventional photosensor circuit 10 jointly form a peak hold circuit against noises, a large-amplitude noise level can-be detected in error as an optical signal L.sub.S, lowering the signal-to-noise (S/N) ratio of the photosensor circuit 10. As a result, the detectable minimum level of illumination (intensity of light) increases, while sensitivity of the photosensor circuit is reduced.